The objective is to characterize and study the pathogenic potential of an acid fast (AF) bacterium repeatedly isolated from an ultrapure water source furnishing drinking water to some of the Institute's animals. The organism has been identified as Mycobacterium chelonae. Growth characteristics were compared in broth media and on solid phase media. Middlebrook 7H9 broth + albumin fraction V and dextrose gave no advantage over BHI or TSB which gave similar results. Nutrient broth was inferior. TGE agar was as effective or more so than TSA, Mueller-Hinton, and Middlebrook 7H9 broth as above + agar and is our standard agar. Sensitivity to several antimicrobials was determined. From these results the growth of M. chelonae on TGE agar containing vancomycin (3 Mug/ml) + polymyxin (10 Mug/ml) was examined. Preliminary results indicate that M. chelonae is not affected by these concentrations. The media (TGE+VP) should prove useful in examining water samples and animal specimens for the presence of M. chelonae. Athymic mice inoculated IV with M. chelonae developed focal granulomas detectable in the liver on post inoculation day (PID) 2 through PID 50. Intensity of lesions peaked by day 7. AF organisms were rarely seen in sections after day 11. AF bacteria were isolated from livers on PID 2, 4, 7 (5/5), 11 (3/5), 14 (1/5), 21 (2/5), 35 (2/5), 50 (0/5). Spontaneous liver lesions will be searched for AF organisms. This work is significant because of the focal lesions of undetermined origin observed in livers of rodents on chronic bioassays conducted under the NTP. Identifying water-borne bacteria and a Mycobacterium sp. in particular as the cause would allow for corrections in management practices.